1. Field of the Invention
The present invention relates to a magneto-optical element which makes use of the Faraday effect and to a magnetic field measurement apparatus for detecting and measuring a magnetic field intensity with use of the magneto-optical element.
2. Description of the Prior Art
Recently, a magnetic field measurement apparatus which combines a magneto-optical element of the Faraday effect with optical fibers is proposed in order to measure a magnetic field intensity optically (K. Kyuma et al., IEEE Journal of Quantum Electronics, Vol. QE-18 No. 10, 1619 (1982)).
In such a magnetic field measurement apparatus as mentioned above, an electrical current is detected by measuring the magnetic field intensity generated around an electrical conductor through which the electrical current flows. This method has a characteristic that electromagnetic induction noises are not detected or electrical insulation is good because light is used for transmission. Therefore, the method is applicable to a transmission and distribution equipment of electric power.
FIG. 1 shows a principle of a method of measuring a magnetic field by using the Faraday effect. In FIG. 1, a magneto-optical element 1 is placed in a magnetic field H. A light which has been polarized linearly by a polarizer 2 is transmitted through the magneto-optical element. The polarization plane is subjected to rotation in proportion to the magnetic field intensity H according to the Faraday effect. The linearly polarized light which has been subjected to rotation transmits an analyzer 3 which is arranged to change the polarization direction of transmitted polarized light by 45.degree., and the magnitude of the Faraday rotation angle .THETA. is converted to a change in light intensity. The optical output P.sub.out in this measurement is given by the following Equations (1) and (2): EQU P.sub.out =K(1+sin 2.THETA.), (1)
and EQU .THETA.=CHL, (2)
wherein K is a proportional constant, L is a length (cm) of a crystal of the magneto-optical element 1 (refer FIG. 1), the Faraday rotation angle .THETA. is expressed in unit of degree and C is a sensitivity constant in unit of degree/cm.multidot.Oe which expresses the sensitivity of the magneto-optical element 1.
It is proposed to arrange a plurality of magnetic field measurement apparatus with use of the above-mentioned principle at points along a transmission and distribution line to send their electrical outputs of the apparatuses to a computer wherein for example the zero phase current in the transmission and distribution line is detected with use of a reference signal obtained by adding or subtracting the waveforms of the electrical outputs and the occurrence of accidents in the transmission and distribution line is judged.
As a magneto-optical sensor which can be used in such a magnetic field measurement element, a rare earth iron garnet crystal is disclosed which can be expressed by a following chemical formula (3), wherein 1.0.ltoreq.X.ltoreq.1.4 and 0.1.ltoreq.Y.ltoreq.0.7 (U.S. Pat. No. 5,075,546): EQU Bi.sub.X Gd.sub.Y Y.sub.3-(X+Y) Fe.sub.5 O.sub.12. ( 3)
In the prior art rare earth iron garnet crystal, yttrium is substituted partially by bismuth in order to increase the Faraday rotation angle .THETA. or the sensitivity C, while yttrium is substituted partially by gadolinium in order to improve the temperature characteristics. As shown in FIG. 2, the temperature change in sensitivity constant C of the prior art crystals is within .+-.2.5% in a temperature range between -20.degree. C. and +80.degree. C.
However, a magnetic field measurement apparatus constructed with use of the prior art magneto-optic element has problems practically on the temperature characteristics, the sensitivity and the precision of measurement. At present, no magneto-optical element of bismuth substituted rare earth iron garnet crystal can be used practically for a magnetic field measuring apparatus due to bad temperature characteristics. Further, if a light source of 0.8 .mu.m band is used for a prior art bismuth substituted rare earth iron garnet crystal, the output light intensity of the magnetic measurement apparatus becomes small due to the loss of optical absorption in a crystal, and the SN ratio decreases to worsen the precision.
Therefore, if a magneto-optical element which has a smaller loss of optical absorption than the prior art bismuth substitution rare earth iron garnet crystal is provided, the SN ratio can be improved and the measurement precision can be improved. Further, if a short wavelength light source of 0.8 .mu.m band is used, the Faraday rotation angle .THETA. becomes larger than in case of 1.3 .mu.m band, so that the sensitivity constant C of the crystal increases. If the sensitivity constant C increases, the thickness of an element to be used can be decreased, and the time required for crystal growth can be shortened with use of a liquid phase epitaxy technique or a gas phase crystal growth technique.